1. Field of the Invention
The present invention relates to a movable yoke-type lifting magnet device and, more particularly, relates to a device wherein a yoke movable relative to a core does not receive an upwardly and forcibly acting reaction force from a workpiece to be lifted.
2. Description of the Prior Art
Movable yoke-type lifting magnet devices are known as taught in Japanese Utility Model Publications SHO Nos. 61-122279 and 51-126570. The former publication discloses a yoke which is movable relative to a core assembly and which is forcibly driven downwards by a spring. The latter publication discloses a yoke which is movable relative to a core assembly and which is moved by means of weight of the yoke itself. Between the movable yoke and support members for slidably supporting the movable yoke, a clearance is provided for allowing the yoke to move. Accordingly, the movable yoke can tilt a little due to the clearance. In either device of both publications, a lower end of the movable yoke is constantly positioned lower than a lower surface of the core assembly. Therefore, when the device approaches a workpiece to be lifted from an upper side of the workpiece, at first the yoke is brought into contact with the workpiece, and then the yoke is forcibly moved upwards relative to the core assembly, receiving an upwardly acting reaction force from the workpiece.
However, when the upper surface of the workpiece is inclined from a horizontal plane, the direction of the reaction force acting on the yoke from the workpiece is inclined from an exactly vertical direction and the yoke is liable to tilt. As a result, the frictional force caused between the yoke and the support members therefor becomes large. This deteriorates smooth movement of the yoke relative to the core via the support members, injures and deforms the workpiece to be lifted, causes a severe abrasion of the sliding surfaces of the lifting magnet device, and causes unstable magnetic coupling between the yoke and the workpiece. Such troubles often take place when the yoke is moved upwards relative to the core by a large distance and at high speeds.